A cross-sectional structure of a conventional electric water heater is shown in FIG. 18. In the figure, the main body 531 of the electric water heater is provided with a nearly cylindrical container 532 having a heater 533 serving as a heating device at the bottom. Below the container 532, a centrifugal pump 534 serving as a water conveyance device is provided, one end of which is connected to an outlet 535 provided at the bottom of the container 532 and the other end of which is connected to a transfer pipe 536. At the bottom face of the container 532, a temperature detection device 537 for detecting the temperature of the hot water in the container 532 is provided. On the upper face of the main body 531, an operation section 538 having an operation button and a display section is provided. A control circuit 539 controls the heater 533 and the centrifugal pump 534 corresponding to signals from the operation section 538.
The operation of the conventional electric water heater configured as described above will be described below. First, a predetermined amount of water is supplied into the container 532. When connected to power source, the control circuit 539 starts to supply electric power to the heater 533. When the temperature detection device 537 detects the boiling of water, the control circuit 539 stops supplying electric power to the heater 533 once and enters a warming operation state wherein hot water in the container 532 is warmed at a predetermined temperature while electric power supply to the heater 533 is controlled according to a signal from the temperature detection device 537.
When a user who wishes to use hot water operates the operation button of the operation section 538, the centrifugal pump 534 is driven by the control of the control circuit 539, and the hot water in the container 532 is supplied via the transfer pipe 536. When the operation of the operation button is stopped after a desired amount of hot water is supplied, hot water supply stops.
In order to recognize the water level in the container 532, the transfer pipe 536 is generally formed of a transparent glass pipe and the transfer pipe 536 is made visible from outside, whereby the amount of the hot water is confirmed. When the amount of the hot water has reduced, the user supplies water into the container 532.
In the above-mentioned conventional configuration, the water-heating container also functions as a water storage container. Hence, it is usually necessary to store a large amount of hot water, for example, several times the amount of hot water to be used one time (about 100 to 500 cc). Even when a small amount of hot water is required, a large amount of hot water being stored is heated and warmed at all times, thereby causing a problem of consuming electric power wastefully. When an upper lid 545 is opened for water replenishment, a problem of requiring caution for the scattering of hot water having a high temperature, steam and the like is also caused.
As an electric water heater configured to solve the above-mentioned problems, an electric water heater having two tanks, that is, a water storage tank and a water-heating tank, is available. When the hot water in the water-heating tank has reduced, water is automatically supplied from the water storage tank. In this electric water heater, only the amount of water required for one-time use is heated and warmed in the water-heating tank. Hence, the water heater is characterized in that power consumption is reduced. Detection as to whether water is present or not in the water storage tank is carried out by using a method in which a pump for supplying water from the water storage tank to the water-heating tank is operated and a judgment as to whether water is present or not is made depending on the load of the pump. Therefore, periodical pump operation is necessary in order to detect whether water is present or not in the water storage tank.
Since the pump is operated periodically, problems are caused, that is, noise is generated when the pump is operated, and the life of the pump is shortened.
In the above-mentioned electric water heater provided with a plurality of containers of water-heating tank and water storage tank, water is supplied from the water supply container when the hot water in the water-heating container is reduced. In order that hot water shortage is not caused, a method is proposed in which a small amount of water is supplied each time a small amount of hot water is used. Another water heater is also available in which the amount of hot water in the water-heating container is detected by a water amount detection section and water supply is started when the amount of hot water remaining in the water-heating container becomes less than a predetermined amount of hot water.
In this configuration, when the remaining amount of hot water becomes less than the predetermined amount, water supply is started immediately. Hence, if the remaining amount of hot water becomes less than the predetermined amount while the user continuously discharges hot water, water supply begins immediately. This causes an inconvenience of discharging hot water having a lower temperature.
A method of preventing the temperature drop of hot water by interrupting water supply during hot water discharge is also available. However, when hot water is discharged intermittently at short time intervals, for example, when hot water is poured into a plurality of cups of noodles, water supply is carried out between the last hot water discharge and the next hot water discharge. This causes an inconvenience of discharging hot water having a lower temperature.
In the above-mentioned electric water heater provided with the water-heating container and the water supply container holding water to be supplied automatically to this water-heating container so that hot water is not exhausted even in the case when the consumption amount of hot water is large, water is supplied from the water supply container until the water-heating container is filled, each time hot water is used, so that the hot water in the water-heating container is not exhausted by the reduction of hot water.
In this case, since water is supplied to the full level immediately after hot water is used, the user must wait until the whole water supplied to the full level is boiled when an actually required amount of water is more than the full-level amount of water as a matter of course and even when an actually required amount of water is considerably less than the full-level amount of water. Consequently this has a problem of extending the waiting time and lowering usability. For example, in the case when hot water is desired to be poured into six cups of noodles from an electric water heater having a full-level capacity capable of pouring hot water into five cups of noodles, hot water is poured into the five cups of noodles, first. Then, after water is supplied to the full level by automatic water supply, a wait is required until water is boiled, and hot water must be poured into the remaining one cup of noodles. In this case, extra hot water for four cups of noodles is heated, whereby a considerable time difference exists between the time when hot water is poured into the first five cups of noodles and the time when hot water is poured into the last one cup of noodles, including the time for water supply. As a result, the eating start time of only one person is delayed considerably.
In the case when hot water is drained and the electric power source is turned off in the evening for energy conservation, and water is supplied and heated for one cup of coffee in the next morning, in the above-mentioned conventional electric water heater, a considerable time is required until boiling is completed from heating is carried out after water is supplied automatically to the full level. This is caused by that a large amount of hot water is required to be heated even in the case when only one cup of coffee is desired, thereby causing a problem of extending a waiting time.